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metals become perfect conductors (superconductors) but 1 only at temperatures too low to be really useful. The press has given considerable publicity to the unexpected discovery of new superconductors at more accessible temperatures2, and the determined efforts by scientists to go ever higher. To do this it is necessary to understand the structure of these materials (arrangement of the atoms), and this is where the neutrons come in. ©1996 - Institut Laue-Langevin Superconductors and neutrons at Grenoble What is a superconductor ? When electric current circulates in a normal conductor, there is a loss of energy and emission of heat because of its electrical resistance. Copper, the best conductor known at ambient temperature, still has a resistance which is not negligible, and this is why EDF tries to limit losses by building enormous high voltage lines. With superconductors low voltage would be sufficient, as they have no electrical resistance and there would therefore be no loss of energy. Unfortunately very low temperatures are expensive to maintain! produced, creating opposing forces, which repel the magnetic surface. A vehicle gliding in this way without friction over a magnetic surface, rather than on wheels, would need much less energy to move. It is clear that 'high temperature' superconductors have all sorts of potential applications. They are already present in scanners using nuclear magnetic resonance (NMR), which are increasingly replacing X-rays in hospitals for the examination of the interior of the human body. But it will no doubt take years to develop magnetic suspension trains and loss-free electrical cables. The quest for new superconductors These new superconductors were not discovered by chance, but using ideas and theories resulting from fundamental research. New materials Figure 1. Superconductor in levitation above a magnetic surface. Electricity authorities loses 10% of the electricity produced because the metal of the electricity cables heats up as the current passes. This phenomenon, known as the Joule effect, is sometimes useful (electric radiator) but is often a considerable disadvantage. Can it be avoided ? Yes and no. Since 1911 it has been known that some In 1986, G. Bednorz and K.A. Müller in Zurich discovered that a new class of materials first synthetized by B. Raveau in France become superconductors at a much higher temperature, above that of liquid air. This is still cold (- 190°C), but liquid air can be produced and stored relatively inexpensively. Fig. 1 shows a cold superconductor floating above a magnetic surface. As soon as the magnetic force field penetrates the superconductor, electrical currents are 1 2 Lower than -250 ºC Temperatures were achieved of - 238°C (BaLaCuO), then 183°C (YBa2Cu3O7). The current record is - 148°C Figure 2. The temperature of the superconducting transition (Tc) in YBaCuO depends on its oxygen composition (x). The superconductor YBaCuO is a mixture of four atoms (e.g. yttrium, barium, copper, oxygen), with properties which are remarkably sensitive to slight variations in crystal structure. Figure 2, for example, shows how the superconducting temperature depends on the proportion of oxygen. It is obviously important to understand why. differently with atoms and does not care whether they are light or heavy. Fig. 3 shows that neutrons are necessary to see lightweight atoms, such as oxygen in superconductors. And where do neutrons come in ? Neutrons, yes but how ? X-rays, the electron microscope and neutrons are the three techniques most used to examine the atomic structure of materials. Which is the most appropriate for the study of superconductors ? The experiments carried out on superconductors are relatively simple (Fig. 4). The ILL high flux reactor is used to produce beams of neutrons and a precise energy or wavelength is selected by a monochromator crystal and aimed at the sample. Figure 3. Apparent size (visibility) of the different atoms of a superconductor, as seen by X-rays, an electron microscope and neutrons. Figure 4. Diagram of a neutron scattering experiment using a powder Oxygen, whose rôle in superconductivity is so important, is a lightweight atom barely "visible" to Xrays, especially because it is associated here with much heavier atoms (copper, yttrium, barium). In the human body the heavy atoms are concentrated in the bones, and a medical X-ray clearly shows that the lightweight atoms (skin, muscles) appear "transparent" to X-rays. Neutrons are another type of radiation which interacts The scattered neutrons are collected by a detector, and the variations in intensity are analysed as a function of angle by computers, to deduce the atomic arrangement, and to situate the oxygen in the superconductor. oxygen, and on its position in the structure (Fig. 5). This work by a joint ILL-CNRS team resulted in the experiment most frequently cited in the year following the discovery of the superconductor YBaCuO. Since then, scientists from the Bell Labs in the USA working in Grenoble with the CNRS and ILL have shown - an essential point - that the superconductivity temperature depends directly on the Cu-O distances which measure the state of oxidation of the copper. Figure 5. Atomic structure of the superconductor YBaCuO. It shows the copper oxide chains and planes which are now wellknown. Provisional conclusion ... What have we discovered with neutrons ? At ILL we have shown that the oxidation of copper in the superconductor plays an essential rôle: the superconductivity depends on the precise quantity of These neutron experiments have thus enabled us to understand what is important in superconducting oxides. They have shown the direction of research needed to obtain better materials, but there is still a great deal to do.